1. Field of the Invention
The present invention relates to electric vehicles, such as pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs), in particular hybrid electric vehicles having a fuel cell and a secondary cell, that run by converting a dc power from a battery pack, which is a driving power source mounted on such electric vehicles, into an ac power using an inverter and supplying the ac power to a motor. Moreover, the present invention particularly relates to a technique for preventing moving contacts of a positive side main contactor and a negative side main contactor that are provided between the battery pack and the inverter from being welded simultaneously.
2. Description of the Related Art
In so-called hybrid electric vehicles (HEVs) and the like that carry an engine and a motor, there is provided, between a battery pack, which is formed by combining a plurality of secondary cells, and an inverter, which is a power drive, a pair of contact switching devices (hereinafter, referred to as “main contactors”) for conducting/interrupting electric power from the battery pack. These main contactors include a positive side main contactor that is provided between a positive electrode terminal of the battery pack and a high potential input terminal of the inverter and a negative side main contactor that is provided between a negative electrode terminal of the battery pack and a low potential input terminal of the inverter. See JP 2003-36776A, for example.
FIG. 8 is a lateral cross-sectional view showing the configuration of a main contactor. In FIG. 8, the main contactor is constituted by a pair of terminals 81 and 81 that are threaded. Ring terminals of wiring harnesses can be screwed thereto onto the terminal 81. The main contactor also includes a pair of terminal electrodes 81a and 81a that are brazed to tips of the pair of terminals 81 and 81, a moving contact 82 for contacting the pair of terminal electrodes 81a and 81a, a shaft spring 83 and an electromagnetic coil 84 for moving the moving contact 82 forward and backward with respect to the pair of terminal electrodes 81a and 81a. 
Main contactors having such a configuration are mounted on an electric vehicle, such as a HEV, as a pair of positive side and negative side main contactors, with the directions in which the moving contacts are moved forward and backward being identical. When the main contactors are mounted on the HEV or the like, the two functions below are required for the case that the moving contacts are closed and for the case that the moving contacts are open.
First, in the case where the HEV is running and the moving contacts are closed, when the moving contacts bounce upon impact on the main contactors due to an external force (e.g., when the vehicle goes over a bump, or a collision of the vehicle), an arc occurs between the terminal electrodes and the moving contacts (hereinafter, these are simply referred to as “contacts”), and thus a part of the contacts is melted. When the contacts come into contact again, the contacts are cooled and fixed (short-circuited), and become incapable of performing the function of interrupting electric power. In order to prevent this, it is necessary to push the moving contacts against the terminal electrodes with a strong electromagnetic force.
On the other hand, in the case where the HEV is being parked and the moving contacts are open, when the contacts come into contact upon impact on the main contactors due to an external force (e.g., when another vehicle collides with the HEV), an excessive inrush current (e.g., an inrush current of 1200 A when assuming that the terminal voltage of the battery pack is 360 V and the internal resistance of the battery pack is 0.3 Ω) relative to the allowable current for the contacts flows from the battery pack to a large-capacity smoothing capacitor that is provided in the inverter. Thus the contacts are welded and become incapable of performing the function of interrupting electric power. In order to prevent this, it is necessary to push the moving contacts down off the terminal electrodes with a strong spring force.
As described above, for conventional main contactors mounted on electric vehicles such as HEVs, it was necessary to enhance both of two conflicting functions in such a way as to increase the electromagnetic force of the electromagnetic coil as well as increase the spring force of the shaft spring, in order to provide resistance against external impact for the main contactors, and thus there were problems that the sizes of the electromagnetic coil and the shaft spring are increased and the cost is increased.